using WebRequest.CustomRequestHeaders results in crash

[susares]

setting WebRequest.CustomRequestHeaders results in a crash for any caller upon WebRequest.open.
This is because the headers must be injected when the request is in state 'open'.

playground link
=> uncommenting WebRequest... line will result in an error

• Expected result: custom headers should be injected
• Current result: custom headrs won't be injected but result in a crash

[susares]

There might be also an issue with WebRequest.CustomRequestModifiers.
They are applied before the state is open.
If someone uses a CustomRequestModifier to inject headers it would result in the same crash.

simply moving the 'update' calls to where I moved the _injectCustomHeaders section. might not give expected results due to Modifiers that want to set attributes thats should be used by i.e. 'open'

[deltakosh]

Thanks for the fix!